Insulin-like growth factor 1 (IGF-1) also known as somatomedin C or mechano growth factor is a protein that in humans is encoded by the IGF1gene.[1][2] IGF-1 has also been referred to as a "sulfation factor"[3] and its effects were termed "nonsuppressible insulin-like activity" (NSILA) in the 1970s.

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IGF-1 is produced primarily by the liver as an endocrine hormone as well as in target tissues in a paracrine/autocrine fashion. Production is stimulated by growth hormone (GH) and can be retarded by undernutrition, growth hormone insensitivity, lack of growth hormone receptors, or failures of the downstream signalling pathway post GH receptor including SHP2 and STAT5B. Approximately 98% of IGF-1 is always bound to one of 6 binding proteins (IGF-BP). IGFBP-3, the most abundant protein, accounts for 80% of all IGF binding. IGF-1 binds to IGFBP-3 in a 1:1 molar ratio.

In rat experiments the amount of IGF-1 mRNA in the liver was positively associated with dietary casein and negatively associated with a protein free diet.[5]

Deficiency of either growth hormone or IGF-1 therefore results in diminished stature. GH-deficient children are given recombinant GH to increase their size. IGF-1 deficient humans, who are categorized as having Laron syndrome, or Laron's dwarfism, are treated with recombinant IGF-1. In beef cattle, circulating IGF-I concentrations are related to reproductive performance.[6]

IGF-1 binds to at least two cell surface receptors: the IGF-1 receptor (IGF1R), and the insulin receptor. The IGF-1 receptor seems to be the "physiologic" receptor - it binds IGF-1 at significantly higher affinity than the IGF-1 that is bound to the insulin receptor. Like the insulin receptor, the IGF-1 receptor is a receptor tyrosine kinase - meaning it signals by causing the addition of a phosphate molecule on particular tyrosines. IGF-1 activates the insulin receptor at approximately 0.1x the potency of insulin. Part of this signaling may be via IGF1R/Insulin Receptor heterodimers (the reason for the confusion is that binding studies show that IGF1 binds the insulin receptor 100-fold less well than insulin, yet that does not correlate with the actual potency of IGF1 in vivo at inducing phosphorylation of the insulin receptor, and hypoglycemia)..

IGF-1 is produced throughout life. The highest rates of IGF-1 production occur during the pubertal growth spurt. The lowest levels occur in infancy and old age.

Other IGFBPs are inhibitory. For example, both IGFBP-2 and IGFBP-5 bind IGF-1 at a higher affinity than it binds its receptor. Therefore, increases in serum levels of these two IGFBPs result in a decrease in IGF-1 activity.

IGF-1 is closely related to a second protein called "IGF-2". IGF-2 also binds the IGF-1 receptor. However, IGF-2 alone binds a receptor called the "IGF II receptor" (also called the mannose-6 phosphate receptor). The insulin growth factor-II receptor (IGF2R) lacks signal transduction capacity, and its main role is to act as a sink for IGF-2 and make less IGF-2 available for binding with IGF-1R.
As the name "insulin-like growth factor 1" implies, IGF-1 is structurally related to insulin, and is even capable of binding the insulin receptor, albeit at lower affinity than insulin.

DAF-2 is the only insulin/IGF-1 like receptor in the worm. Insulin/IGF-1-like signaling is conserved from worms to humans. DAF-2 acts to negatively regulate the forkhead transcription factor DAF-16 through a phosphorylation cascade. Genetic analysis reveals that DAF-16 is required for daf-2-dependent lifespan extension and dauer formation. When not phosphorylated, DAF-16 is active and present in the nucleus.

Factors that are known to cause variation in the levels of growth hormone (GH) and IGF-1 in the circulation include: genetic make-up, the time of day, age, sex, exercise status, stress levels, nutrition level and body mass index (BMI), disease state, race, estrogen status and xenobiotic intake.[9] The later inclusion of xenobiotic intake as a factor influencing GH-IGF status highlights the fact that the GH-IGF axis is a potential target for certain endocrine disrupting chemicals - see also endocrine disruptor.

Rare diseases characterized by inability to make or respond to IGF-1 produce a distinctive type of growth failure. One such disorder, termed Laron dwarfism does not respond at all to growth hormone treatment due to a lack of GH receptors. The FDA has grouped these diseases into a disorder called severe primary IGF deficiency. Patients with severe primary IGFD typically present with normal to high GH levels, height below -3 standard deviations (SD), and IGF-1 levels below -3SD. Severe primary IGFD includes patients with mutations in the GH receptor, post-receptor mutations or IGF mutations, as previously described. As a result, these patients cannot be expected to respond to GH treatment.

The IGF signaling pathway appears to play a crucial role in cancer. Several studies have shown that increased levels of IGF lead to an increased risk of cancer.[10] Studies done on lung cancer cells show that drugs inhibiting such signaling can be of potential interest in cancer therapy.[11]

Interpretation of IGF-1 levels is complicated by the wide normal ranges, and variations by age, sex, and pubertal stage. Clinically significant conditions and changes may be masked by the wide normal ranges. Sequential management over time is often useful for the management of several types of pituitary disease, undernutrition, and growth problems.

Mecasermin (brand name Increlex) is a synthetic analog of IGF-1 which is approved for the treatment of growth failure.[13] IGF-1 has been manufactured recombinantly on a large scale using both yeast and E. coli.

Several companies have evaluated IGF-1 in clinical trials for a variety of additional indications, including type 1 diabetes, type 2 diabetes, amyotrophic lateral sclerosis (ALS aka "Lou Gehrig's Disease"), severe burn injury and myotonic muscular dystrophy (MMD). Results of clinical trials evaluating the efficacy of IGF-1 in type 1 diabetes and type 2 diabetes showed great promise in reducing hemoglobin A1C levels, as well as daily insulin consumption. However, the sponsor, Genentech, discontinued the program due to an exacerbation of diabetic retinopathy [citation needed] in patients coupled with a shift in corporate focus towards oncology. Cephalon and Chiron conducted two pivotal clinical studies of IGF-1 for ALS, and although one study demonstrated efficacy, the second was equivocal, and the product has never been approved by the FDA.

However, in the last few years, two additional companies Tercica and Insmed compiled enough clinical trial data to seek FDA approval in the United States. In August 2005, the FDA approved Tercica's IGF-1 drug, Increlex, as replacement therapy for severe primary IGF-1 deficiency based on clinical trial data from 71 patients. In December 2005, the FDA also approved Iplex, Insmed's IGF-1/IGFBP-3 complex. The Insmed drug is injected once a day versus the twice-a-day version that Tercica sells.

Insmed was found to infringe on patents licensed by Tercica, which then sought to get a U.S. district court judge to ban sales of Iplex.[14] To settle patent infringement charges and resolve all litigation between the two companies, Insmed in March 2007 agreed to withdraw Iplex from the U.S. market, leaving Tercica's Increlex as the sole version of IGF-1 available in the United States.[15]

By delivering Iplex in a complex, patients might get the same efficacy with regard to growth rates but experience fewer side effects with less severe hypoglycemia[citation needed]. This medication might emulate IGF-1's endogenous complexing, as in the human body 97-99% of IGF-1 is bound to one of six IGF binding proteins[citation needed]. IGFBP-3 is the most abundant of these binding proteins, accounting for approximately 80% of IGF-1 binding.

In a clinical trial of an investigational compound MK-677, which raises IGF-1 in patients, did not result in an improvement in patients' Alzheimer's symptoms.[16] Another clinical demonstrated that Cephalon's IGF-1 does not slow the progression of weakness in ALS patients. Previous shorter studies had conflicting results.[17]

IGFBP-3 is a carrier for IGF-1, meaning that IGF-1 binds IGFBP-3, creating a complex whose combined molecular weight and binding affinity allows the growth factor to have an increased half-life in serum. Without binding to IGFBP-3, IGF-1 is cleared rapidly through the kidney, due to its low molecular weight. But when bound to IGFBP-3, IGF-1 evades renal clearance. Also, since IGFBP-3 has a lower affinity for IGF-1 than IGF-1 has for its receptor, IGFR, its binding does not interfere with IGF-1 function. For these reasons, an IGF-1/IGFBP-3 combination approach was approved for human treatment... brought forward by a small company called Insmed. However, Insmed fell afoul patent issues, and was ordered to desist in this approach.

IGF-1 has also been shown to be effective in animal models of stroke when combined with Erythropoietin. Both behavioural and cellular improvements were found.[18]